1. Field of the Invention
The present process is directed to the removal of acrylonitrile monomer from aqueous dispersions of acrylonitrile polymers and, more particularly, to the removal of acrylonitrile monomer by the addition of methanol to the aqueous dispersion and thereafter distilling a methanol-acrylonitrile azeotrope from the dispersion until the level of free acrylonitrile therein is reduced to less than 10 and usually less than 1.0 ppm.
2. Description of the Prior Art
U.S. Pat. No. 3,480,516 issued on Nov. 25, 1969 discloses a process for separating nitroalkanes from alkanes by adding methanol to the mixture and thereafter distilling to remove a methanol-alkane azeotrope. European Patent Application No. 0028348 published May 13, 1981 discloses removing free acrylonitrile from acrylonitrile containing polymers in the form of an aqueous dispersion or emulsion by azeotropic distillation with water. However, it has been recognized that such a distillation allows residual amounts of free acrylonitrile to remain in the system. The aforementioned application teaches that after water stripping the residual acrylonitrile may be further reduced by adding additional catalyst and comonomers in excess of the amount of free acrylonitrile present after at least 90% of the monomers have been polymerized.
U.S. Pat. No. 4,251,412 issued on Feb. 17, 1981 discloses the removal of residual acrylonitrile from acrylonitrile polymers in aqueous systems by addition of an amine to the system.
It is known that acrylonitrile and methanol form an azeotrope which boils at 61.4.degree. C. at atmospheric pressure and contains 38.7% by weight acrylonitrile. The use of this azeotrope to separate acrylonitrile from water and thereby permit analysis of small amounts of acrylonitrile is discussed in "Determination of Small Amounts of Acrylonitrile in Aqueous Industrial Streams" Daues et al., Analytical Chemistry, Vol. 29, No. 7, July 1957, pp. 1035-37.